Control of airflow in a uniflow-scavenged, two-stroke cycle, opposed-piston engine during transient operation

Abstract

Control of airflow in a uniflow-scavenged, two-stroke cycle, opposed-piston engine during transient operation includes monitoring at least one operating parameter of the engine to recognize a transition to a transient state of engine operation. If a transient state of operation is detected, fuel injection and airflow into to the cylinders of the engine are controlled to optimize combustion and limit emissions. Airflow into cylinders of the engine may be controlled by increasing a scavenging ratio of the engine or by increasing a trapping efficiency of the engine.

The invention claimed is: 
     
       1. A method of controlling a uniflow-scavenged, two-stroke cycle, opposed-piston engine, comprising:
 monitoring a transient indication parameter of the engine; 
 determining, based on the transient indication parameter, whether the engine is in a transient mode of operation; 
 when the engine is in a transient state of operation:
 controlling fuel injection into cylinders of the engine by changing one or more of a common-rail pressure and a fuel injection duration; 
 controlling unidirectional airflow through the cylinders of the engine by increasing a scavenging ratio of the engine or by increasing a trapping efficiency of the engine; 
 determining when the transient state of operation ends; and, 
 transitioning the engine to a steady state of operation: 
 
 otherwise, operating the engine in a steady state of operation when the engine is not in a transient state of operation. 
 
     
     
       2. The method of  claim 1 , wherein the transient indication parameter comprises an accelerator position. 
     
     
       3. The method of  claim 1 , wherein controlling unidirectional airflow through the cylinders of the engine comprises changing one or more of a supercharger shunt valve setting, a supercharger drive ratio setting, and a turbine vane setting. 
     
     
       4. The method of  claim 1 , wherein increasing the scavenging ratio of the engine comprises:
 decreasing an exhaust backpressure of the engine; and, 
 increasing a velocity of unidirectional airflow through the cylinders of the engine. 
 
     
     
       5. The method of  claim 4 , wherein increasing the scavenging ratio of the engine further comprises, after increasing the scavenge ratio for a calibration period, increasing a compressor outlet pressure of the engine. 
     
     
       6. The method of  claim 1 , wherein increasing the trapping efficiency of the engine comprises:
 decreasing an exhaust backpressure of the engine; 
 increasing a velocity of unidirectional airflow through the cylinders of the engine; and, 
 increasing a compressor outlet pressure of the engine. 
 
     
     
       7. The method of  claim 6 , wherein increasing the trapping efficiency of the engine further comprises, after increasing the trapping efficiency for a calibration period, reducing a compressor outlet pressure of the engine. 
     
     
       8. The method of  claim 1 , wherein the engine comprises active air handling devices including at least one valve, a supercharger drive, and a variable geometry turbine, and transitioning the engine to the steady state of operation comprises issuing a transient command θ 2  for an actuator of at least one air handling device from an engine control unit (ECU), monitoring an elapse of time from when the transient command θ 2  was issued, and transitioning the engine to the steady state of operation in response to an elapse of a calibration time. 
     
     
       9. The method of  claim 8 , wherein the engine comprises a steady state control process in which an airflow parameter comprising one of mass airflow, boost pressure, exhaust and back-pressure in the air handling system is sensed and an error value is determined by subtracting the sensed parameter value from a desired set-point value for the airflow parameter, and transitioning the engine to the steady state of operation comprises transitioning the engine to the steady state of operation when:
 an elapse of a calibration time occurs; or, 
 the error value is less than a calibration value. 
 
     
     
       10. The method of  claim 9 , wherein controlling unidirectional airflow through cylinders of the engine comprises changing one or more of a supercharger shunt valve setting, a supercharger drive ratio setting, and a turbine vane setting. 
     
     
       11. A method of controlling an air handling system of a uniflow-scavenged, two-stroke cycle, opposed-piston engine equipped with at least one cylinder with a bore and axially-spaced exhaust and intake ports that communicate with the bore, a pair of pistons disposed in opposition in the bore and operative to open and close the exhaust and intake ports during operation of the engine, the air handling system including a charge air subsystem to provide charge air to the intake port, an exhaust subsystem to receive exhaust gas from the exhaust port, and a supercharger operable to pump charge air in the charge air subsystem, the method comprising:
 monitoring a transient indication parameter of the engine; 
 determining, based on the transient indication parameter, whether the engine is in a transient state of operation; 
 when the engine is in a transient state of operation:
 at an onset of the transient state of operation, opening a backpressure valve in the exhaust subsystem to reduce backpressure resistance to airflow through the air handling system; 
 at the onset of the transient state of operation, controlling unidirectional airflow through cylinders of the engine by changing a supercharger shunt valve setting to increase a supercharger pressure ratio of the engine or by changing a supercharger drive ratio setting to increase the supercharger pressure ratio of the engine; 
 determining when the transient state of operation ends; and 
 then, transitioning the engine to a steady state of operation; 
 
 otherwise, operating the engine in the steady state of operation if the engine is not in the transient state of operation. 
 
     
     
       12. The method of  claim 11 , wherein the transient indication parameter comprises an accelerator position or an engine load. 
     
     
       13. The method of  claim 11 , wherein the engine further includes a turbocharger with a turbine in the exhaust subsystem and a compressor in the charge air subsystem, upstream of the supercharger, and controlling unidirectional airflow through cylinders of the engine further comprises one or more of decreasing an exhaust backpressure of the air handling system, and increasing a compressor outlet pressure of the air handling system. 
     
     
       14. An airflow control combination for a uniflow-scavenged, two-stroke cycle, opposed-piston engine equipped with at least one cylinder with a bore and axially-spaced exhaust and intake ports that communicate with the bore, a pair of pistons disposed in opposition in the bore and operative to open and close the exhaust and intake ports during operation of the engine, and an air handling system including a charge air subsystem to provide charge air to the intake port, an exhaust subsystem to receive exhaust gas from the exhaust port, a supercharger operable to pump charge air in the charge air subsystem, and a command-controlled shunt valve which promotes a charge air pressure ratio across the supercharger, the airflow control combination comprising:
 a sensor that senses one of engine acceleration and engine load of the engine; 
 a sensor that detects charge air pressure at the intake of the supercharger; 
 a sensor that detects charge air pressure at the outlet of the supercharger, and, 
 a control unit programmed to:
 determine an occurrence of a torque demand for the engine, the torque demand having an intensity based on an intensity of a rate of change of engine acceleration or engine load with respect to a transient intensity threshold value; 
 produce a transient command for the shunt valve to increase a charge air pressure ratio across the supercharger when the intensity of the torque demand exceeds the transient intensity threshold; and 
 produce a steady state command to control the charge air pressure ratio across the supercharger to a desired setpoint when the intensity of the torque demand falls below the transient intensity threshold. 
 
 
     
     
       15. The airflow control combination of  claim 14 , further comprising a backpressure valve in the exhaust subsystem to control a backpressure in the air handling system, in which the control unit is further programmed to produce a transient command to open the backpressure valve when the intensity of the torque demand exceeds the transient intensity threshold. 
     
     
       16. An airflow control combination for a uniflow-scavenged, two-stroke cycle, opposed-piston engine equipped with at least one cylinder with a bore and axially-spaced exhaust and intake ports that communicate with the bore, a pair of pistons disposed in opposition in the bore and operative to open and close the exhaust and intake ports during operation of the engine, a charge air channel to provide charge air to the intake port, an exhaust channel to receive exhaust gas from the exhaust port, a supercharger operable to pump charge air in the charge air channel, and a command-controlled supercharger drive which promotes a charge air pressure ratio across the supercharger, the airflow control combination comprising:
 a sensor that senses one of engine acceleration and engine load of the engine; 
 a sensor that detects charge air pressure at the intake of the supercharger; 
 a sensor that detects charge air pressure at the outlet of the supercharger and, 
 a control unit programmed to:
 determine the occurrence of a torque demand for the engine, the torque demand having an intensity based on an intensity of a rate of change of engine acceleration or engine load with respect to a transient intensity threshold value; 
 produce a transient command to actuate the supercharger drive to increase the charge air pressure ratio across the supercharger when the intensity of the torque demand exceeds the transient intensity threshold; and 
 produce a steady state command to control the charge air pressure ratio across the supercharger to a desired setpoint when the intensity of the torque demand falls below the transient intensity threshold. 
 
 
     
     
       17. The airflow control combination of  claim 16 , further comprising a backpressure valve in the exhaust channel to control a backpressure in the air handling system, in which the control unit is further programmed to produce a transient command to open the backpressure valve when the intensity of the torque demand exceeds the transient intensity threshold. 
     
     
       18. A control process executable by a programmed control unit of a uniflow-scavenged, two-stroke cycle, opposed-piston engine equipped with one or more cylinders, each cylinder having a bore and axially-spaced exhaust and intake ports that communicate with the bore, a pair of pistons disposed in opposition in the bore and operative to open and close the exhaust and intake ports during operation of the engine, an air handling system of the engine including a charge air subsystem to provide charge air to the intake ports, an exhaust subsystem to receive exhaust gas from the exhaust ports, and a plurality of command-controlled air flow devices positioned in the charge air and exhaust subsystems to establish and sustain a unidirectional flow of gas through the cylinders, in which the control process comprises:
 controlling at least one of the plurality of command-controlled airflow devices in a steady state mode of engine operation with a steady-state command θ 1 ; 
 controlling the at least one of the plurality of command-controlled airflow devices in a transient mode of engine operation with a transient command θ 2 ; and, 
 initiating the steady state control mode by issuing the steady-state command θ 1 , and transitioning control of the at least one of the plurality of command-controlled airflow devices to the transient control mode by issuing the transient command θ 2  when an onset of a transient condition of the engine is detected by the programmed control unit. 
 
     
     
       19. The control process of  claim 18 , in which the control process further comprises determining when to transition control of the at least one of the plurality of command-controlled airflow devices from the transient control mode to the steady state control mode. 
     
     
       20. The control process of  claim 19 , in which the controlling the at least one of the command-controlled plurality of airflow devices in a steady state mode comprises generating a set-point correction value (c) to correct a current position of the at least one of the plurality of command-controlled airflow devices, generating a device position command, and adding the set-point correction value (c) and the device position command to generate the steady state command θ 1 . 
     
     
       21. The control process of  claim 20 , in which controlling the at least one of the plurality of command-controlled airflow devices in a transient mode of engine operation comprises generating a transient command θ 2 ′ in response to a level of transient intensity or a rate of change of air/fuel ratio (AFR), and providing, via a gate, the final transient command θ 2  on the basis of elapsed time since the transient condition has been detected. 
     
     
       22. The control process of  claim 21 , in which controlling the at least one of the plurality of command-controlled airflow devices is changed from the transient control mode to steady state control mode when either the transient command θ 2 ′ has been active for a calibration time, or when an error value (e) representing a difference between a desired set-point of an airflow parameter and a sensed value of the airflow parameter is less than a calibration value. 
     
     
       23. The control process of  claim 22 , in which the control process further comprises generating an on-board diagnostic (OBD) fault when the error value (e) is greater than the calibration value after a calibration time, and is not changing. 
     
     
       24. The control process of  claim 19 , in which the plurality of command-controlled airflow devices comprises a supercharger shunt valve of the air handling system, a supercharger drive of the air handling system, a variable geometry turbine of the air handling system, an exhaust backpressure valve of the air handling system, and a wastegate valve of the air handling system.